Aerosol particles may be formed in combustion processes, e.g. when combusting wood, wood pellets, peat, or municipal waste. Aerosol particles may also be formed in industrial processes such as hot galvanization, welding, or glass smelting. Said aerosol particles are often harmful to the environment or health. In particular, so called nanoparticles may cause health problems when inhaled, because they may penetrate into human lungs. Toxic heavy metals vaporized in industrial processes may also be condensed and enriched in nanoparticles. The term nanoparticle refers herein to particle diameters smaller than or equal to 500 nm.
It is known that aerosol particles may be separated from flue gases by using filtration, or by using electrostatic precipitators. Electrostatic precipitators are typically characterized by a low pressure drop and the ability to handle high particle concentrations.
In conventional electrostatic precipitators, particles are typically charged by means of a corona discharge, and the charged particles are displaced to collection plates by means of an electric field. Typically, charging and electrical displacement are arranged to take place in the same volume. In conventional electrostatic precipitators, the aim is to use a high electric field together with a low charge density, because a strong electric field combined with a high charge density would increase energy consumption. Efficient charging of particles in the 1-100 μm regime requires a strong electric field. Conventional electrostatic precipitators are typically optimized for separating particles whose diameters are in the range of 1 to 100 μm.
On the other hand, efficient charging of nanoparticles requires a high charge density in the particle-laden gas. Thus, conventional electrostatic precipitators are typically not very effective and/or economical when the task is to separate nanoparticles.
A problem with Prior Art solutions for cleaning collection plates of an electrostatic precipitator is that particles loosened during the cleaning process may be captured back to the gas stream. This may be avoided if the gas flow is shut off during the cleaning process. However, this may make the gas cleaning system more complex.
Particles may be charged by a corona discharge such that charging takes place separately from the electrical displacement. However, in that case particles may be deposited on all surfaces in the vicinity of the corona electrode, and this can make cleaning of an electrostatic precipitator more difficult.